Natural killer (NK) cells are a subpopulation of large granular lymphocytes (LGL) that are able to recognize and lyse a wide variety of virally infected or neoplastic target cells, as well as certain microorganisms without previous sensitization or major histocompatibility complex (MHC)-restriction. NK cells do not rearrange T-cell receptor genes, and do not express cell surface TcR molecules or components of the cluster-of-differentiation 3 (CD3) transduction complex. NK cells do express signal transduction molecules, including CD2, CD16, and CD45. Among these, only CD45, however, is not NK cell specific. With the use of a monoclonal antibody against rat IL-2 activated NK cells, we have recently identified a 30 kD protein which is highly expressed on NK cells. This protein functions as a disulfide linked dimer capable of mediating transmembrane signalling in NK cells. Furthermore, we have cloned and sequenced the gene encoding this novel signal transduction molecule in the rat. Because of its biological activity and its uniqueness in sequence, this protein may be considered part of a new receptor complex,selectively expressed on NK cells (NKR). Due to the important function that this protein (PI) may have in the NK cell mediated cytotoxic activity, we decided to clone the mouse NKR-P1 homologue. The mouse is a more extensively studied animal model than the rat and allows us to perform experiments that are currently almost impossible to be successfully accomplished in the rat. More recently, by cross-hybridizing the NKR-P1 rat gene to the mouse CDNA present in a library generated from highly purified, interleukin-2 (IL-2) activated, mouse Natural Killer (NK) cells, we succeeded in cloning the mouse homologue of NKR-P1 as both CDNA and genomic genes. Three similar messages, differing in size and sequence, are co-transcribed in the mouse NK cells. By using probes specific for each of the co-expressed transcripts, we were able to map all 3 of these genes to a common segment of chromosome 6 closely linked to the region where NK1.1 is believed to be mapped. The gene encoding NK1.1 antigen has not yet been cloned. On the basis of these results, we now feel that we are in the position of focusing the aims of our project more precisely. These aims will be: 1) to study the function of mouse NKR-P1 gene family by first defining its relationship to the NK1.1 and NK2.1 antigens; 2) to clone the promotor region of all three co-transcribed mouse NKR-P1 genes from the genomic clones we have isolated and attempt to characterize a transacting factor common to all of them; and 3) to generate transgenic mice using constructs of different sizes, derived from the available 5' upstream region segments, to better study the regulation of these genes. We are confident that the information provided by these tests will clarify the physiological role played y NKR-P1 and lead to insights as to how to exploit its function in vivo.